This invention relates to antennas for receiving and/or transmitting radio frequency signals, and in particular, to antennas configured in a space-efficient manner.
It is common for the signal processing components of a radio transceiver to be implemented in a single integrated circuit so as to allow the provision of a radio receiver in small portable devices, such as mobile telephones and media players. However, in order to achieve an acceptable signal quality over the commercial FM band (76 MHz to 108 MHz) it has been necessary to continue to use an external antenna. This is a result of the relatively long wavelength at these frequencies (approximately 3 meters) which is much larger than the typical size of a mobile phone or media player (8 to 15 cm). Simply shrinking a conventional antenna design so that it will fit inside a portable device leads to a very large loss in efficiency and a significant reduction in the bandwidth of the antenna. These problems are discussed in a paper by D. Aguilar et al entitled “Small handset antenna for FM reception”, published in Microwave and Optical Technology Letters, Vol. 50, No. 10 (October 2008). The paper also proposes some improved space-efficient antenna designs based around loop antenna configurations.
For some devices, using an external antenna is not a significant problem since conductive components of the device that are otherwise present to serve another function can be used as the antenna. For example, in a mobile phone that has a wired headset the cable that connects the phone to the headset can be used as an antenna for receiving frequency modulated (FM) broadcast radio signals in the band 76 MHz to 108 MHz. However, it is becoming increasingly common for wireless headsets to be used with mobile phones which do not provide a cable suitable for use as an antenna. Furthermore, radio reception functionality could be added to many more devices if a long antenna wire were not required.
The problem is particularly critical for the relatively long wavelength radio signals in the commercial FM band, but analogous miniaturisation problems apply in other radio frequency bands where it is desirable to embed an antenna within a small integrated circuit or restricted space.
There is therefore a need for a space-efficient antenna structure which allows a radio frequency antenna to be embedded within a device or integrated circuit.